"That's what made them highly susceptible to the virus," said Sarah Matthews, epidemiology program manager at the Orange County Health Department.

Health officials said the 4-year-old attended a local daycare center for one day, but the other three were infectious over winter break when they were off from their private school.

The health department sent a letter to all places the children visited while they were infectious.

The department encourages parents to make sure their children are immunized against measles with the MMR vaccine. The first dose should be given at 12 to 15 months of age, and the second dose should be given at 4 to 6 years of age.

Measles is spread through the air and is highly contagious.

Symptoms include blotchy rash, fever, cough, runny nose, red and watery eyes, feeling run down and tiny white spots with bluish-white centers found inside the mouth.